The present invention generally relates to a negative electrode for gastight alkaline storage batteries, and in particular, to a negative electrode having a carbon-black-containing gas-consumption layer.
It is generally desirable to charge gastight storage batteries with the highest currents possible. To this end, steps have been taken to accelerate oxygen consumption by providing the negative electrodes of such cells with auxiliary electrodes made of a catalytically active material and which are at the same potential as the negative (main) electrode.
German Offenlegungsschrift 35 34 758 shows one such electrode arrangement, for a Ni/Cd button cell. The disclosed cell comprises a negative electrode in tablet form which is pressed into a nickel-wire gauze basket, and which is composed of a carbon-containing mixture of active carbon, conductive carbon black and PTFE (which serves as a binder). The resulting mixture is rolled into the nickel-wire gauze material of the basket lid.
German Offenlegungsschrift 34 33 544 discloses a negative cadmium electrode which is directly coated with a catalytically active carbon material in dry form. Before the dry application of the carbon material (for example, by dusting), the electrode is moistened with a dispersion of a water-soluble cellulose ether (sold, for example, under the tradename "Tylose"). Cellulose ethers of this kind, such as methylcellulose or carboxymethylcellulose (which generally find application as thickeners, binders and suspending agents), in this case serve mainly as adhesives.
However, in a dry coating, certain disadvantages may arise. This is because, on the one hand, only the outer electrode surface and not the inner pores are coated and, on the other hand, irregularities are often present in the coating (thickness) which can impair the electrolyte absorption of the negative electrode. As a result, such cells require long exposure times during their production.
Wet coatings based on aqueous dispersons have also proved to be less than satisfactory because, when the cell is cyclically charged and discharged, the coating loses some of its hydrophobic nature. This results in a deterioration of the coating's gas-consumption kinetics, which are unalterably associated with the maintenance of three-phase boundary layers (including an electrically conducting solid-state surface, a gas phase and the liquid electrolyte). As a consequence, the cell tends to exhibit a high internal pressure, which contributes to a shortening of the service life of the cell. A further disadvantage of known aqueous dispersons is that they have a tendency to demix and separate the carbon component and the PTFE component.